Sheathed cables are regularly used to remotely transmit control actions from mechanical controls to equipment on marine vessels. These control actions, which may be viewed as mechanical commands, are transmitted by the extension or retraction of a wire cable from its sheath in response to the physical displacement of a lever or other type control. For example, the position of a vessel steering wheel is typically transmitted by a sheathed cable that connects the steering wheel to a rudder control mechanism, or to the housing of an outboard motor to control its angle relative to the transom of the vessel. Any change in the orientation of the steering wheel extends or retracts the wire cable from its sheath. This change in wire cable extension moves the rudder or other steerage control mechanism to a desired orientation.
A sheathed cable can also be used to control the speed of a vessel. The sheathed cable is connected between a throttle lever at the navigation station and a throttle on the engine. A displacement in the position of the throttle lever causes an extension or retraction of the wire cable from its sheath. The wire cable displacement is translated by the throttle into a new speed for the vessel engine.
The biggest threats to the physical integrity and smooth operation of wire control cables on a vessel are mechanical damage, wear, and corrosion. To combat these threats, manufacturers of wire cables have developed sophisticated cable sheathing. A sheathed cable is a flexible tube with a movable wire control line extending through the tube and beyond each end. Usually, a control line is constructed of several strands wrapped concentrically around each other to create a single control line. This type of cable is strong, flexible, and resists stretching. The control line can freely extend or retract along the longitudinal axis of the sheath. Typically, a cable sheath has at least three distinct material layers: an interior low friction coating, a middle armored layer, and a sealed exterior layer. The interior layer of the sheath comprises a smooth and flexible material that is in direct contact with the control line. The smooth interior surface is designed to reduce cable binding and wear and to minimize friction relative to movement of the control line. Thus, many manufacturers prelubricate the wire cable and interior surface of the sheath with a grease to reduce friction and prevent the ingress of corrosive materials, such as salt water. The middle layer of the sheath is often constructed with helically wound or coiled steel wire strands. The steel wire strands permit the sheath to remain flexible, but provide resistance to mechanical compression and kinking. The exterior layer of the sheath is a hard but flexible material that resists cuts, abrasions, and the ingress of corrosive materials. The three sheath layers in combination provide a smooth, strong, and flexible longitudinal pathway for a control line.
The control line in an installed sheathed cable is constantly extending and retracting to transmit navigational commands and other control actions. Corrosive materials such as salt spray can be deposited on the surface of the control line as it extends from its sheath. Subsequently, these materials are drawn inside the sheath as the control line is retracted. Once inside the sheath, the corrosive materials begin acting on the control line, increasing the friction that resists its longitudinal movement through the passage in the sheath. An increase in friction will cause the control line to bind and wear excessively. Another consequence is an acceleration of the natural oxidation of the metal strands that comprise the control line. Oxidation of the wire strands will weaken the control line, eventually causing it to break. Also, the ability of the control line to extend and retract with minimal compressibility is impaired as oxidation increases. The deleterious consequences of the ingress of corrosive materials inside a cable sheath can seriously threaten the physical integrity and reliability of a control line, requiring that the sheathed cable be removed and replaced. Removal and replacement of such cables can be relatively labor intensive and costly, because the cables are often routed through restricted access areas on a vessel. These problems also can arise (but usually to a lesser extent) when sheathed cables are used in other non-marine applications.
Wire cable manufacturers recognize that corrosive materials will inevitably be dram inside cable sheaths that are installed on a marine vessel, and that even if corrosion does not occur, the prelubrication will eventually dissipate, causing wear and eventual failure of the sheathed cable. Subsequently, these manufacturers recommend that regularly scheduled lubrication of sheathed cables should be part of a preventive maintenance program. One method used to lubricate a sheathed cable is to remove the sheathed cable from its installation, lubricate the control line inside the sheath, and then reinstall the cable. The expense of regularly removing, lubricating, and reinstalling sheathed cables has created a long felt need in the marine industry (and in other industries in which such cables are used) to lubricate the cables in situ. One solution to this problem might be to introduce a lubricant into the passage within the sheath through a fitting at the ends of a sheathed cable. Unfortunately, the ends of a sheathed cable are not always readily accessible in a marine vessel. The cost to access the ends of a sheathed cable on a marine vessel could easily negate the benefit of lubricating the control line without removing the sheathed cable. Accordingly, there is clearly a need for an alternative approach for injecting lubricant into the passage of a sheathed cable.